1. Field of the Invention
This invention relates to a semiconductor optical amplifier: an optical amplifying apparatus, such as a semiconductor optical amplifier module including a semiconductor optical amplifier and input and output means for connecting thereto; an optical communication system or network using this optical amplifier or amplifying apparatus, and an integrated optical node involving this amplifying apparatus.
2. Related Background Art
Generally, the semiconductor optical amplifier or amplifying apparatus means such as involves a semiconductor laser structure including an active layer and a cladding layer and amplifiers an input light by means of a bias current below a threshold. In the optical communication field, this has been developed as a device for compensating for an optical loss that occurs in optical fibers or at connections between optical fibers.
However, there has been a problem of polarization dependency of optical amplification factor (i.e., the optical amplification factor differs depending on different polarization modes) when such semiconductor optical amplifier is used in the optical fiber communication systems. Generally, the state of polarization of the output light transmitted through the optical fiber is unstable, so the level of output light from the optical amplifier will not be stable when such light from the optical fiber is input into such amplifier having the above polarization dependency. Further, the fluctuation of such output burdens a light receiving system regarding the dynamic range and the like. This is a vital drawback which limits the scale of the communication system.
A single polarization optical fiber can resolve such problem when this fiber is used for optical transmission. But, this resolution raises a cost and results in another problem that such optical fiber does not match with other systems well.
Thus, several attempts have been made for fabricating such an optical amplifier as dissolves the polarization dependency. One method is to dissolve the polarization dependency by making thick the active layer of the semiconductor optical amplifier. Another method is to combine such a device as has the polarization dependency with other optical elements. There are several methods of this type. For instance, Japanese Patent Laid-open No. 1-102983 discloses the method in which a polarization rotator is provided upstream the optical amplifier and Japanese Patent Laid-open No. 1-61079 shows such method in which an input light is resolved into two polarized components, these two lights are respectively input to two optical amplifiers and amplified outputs therefrom are again combined.
However, those prior art methods have respective drawbacks. First, in the method of thickened active layer, there occurs the problem that other characteristics will be degraded. That is, when the active layer is thickened, injection current will increase, leading to degradation of noise characteristic due to the generation of heat and increase of spontaneous emission. Second, in the method of using external optical systems, the number of components will generally increase and the size of its module will be large. Moreover, manufacturing efficiency is low and its cost is relatively high. Particulars thereof will be explained referring to FIG. 1 that illustrates the example in Japanese Patent Laid-open No. 1-61079. In this structure, a signal light transmitted through an optical fiber 93 is resolved into two polarized components by a polarization beam splitter (PBS) 96 and these component lights are input into respective optical amplifiers 92. Thus, those amplifiers 92 must be separately provided and the PBS 96 and mirrors 97 are needed. Further, optical alignment among the optical components should be performed whose accuracy determines the performance of the optical amplifying apparatus, and the optical components should be structured in such a manner that no reflection lights will be generated among the optical components. This makes it necessary to provide antireflection coating on end surfaces of the PBS, etc. Thus, the size will be large where its module is fabricated, and the cost will be high.